The instant invention relates to the protection of electrical apparatus, such as refrigeration compressors, from power source undervoltage conditions, often referred to as brown-outs. Manufacturers usually provide thermal protection devices as part of their equipment; however, these are often inadequate during brown-out conditions especially where voltages may fluctuate in the brown-out range. Single-phase motors are particularly susceptible to damage because of high current draws in their starting windings which are usually disconnected from the line via centrifical switches. It has been found that in many situations factory installed thermal protection devices will not prevent permanent motor damage upon frequent start cycling at below normal line voltages.
Heretofore, some undervoltage devices provide protection by merely assuring time delay by way of timers or thermal devices upon sensing an undervoltage condition. Other such devices, relative to the instant invention, are complex in their circuitory and comprise considerable more expense in components and manufacture.
A primary object of the invention is to provide a device which is operative to disconnect an electrical apparatus from a voltage source upon sensing an occurrence of a preselected undervoltage condition of the source.
Another object of the invention is to provide that the device be adjustable to a selected undervoltage condition.
A still further object of the invention is to provide that the device operates between an adjustable preselected source voltage differential.
A still other object of the invention is to provide a time delay to prevent the device from actuating upon momentary source voltage drops to the selected undervoltage condition.
Still another object of the invention is to provide visual indicia to indicate the device operative.
A still further object of the invention is to provide that upon return of acceptable source voltage the device be operative to reconnect the apparatus to the voltage source.
Generally, the present invention comprehends a device to protect electrical apparatus subject to damage from undervoltage line or source conditions. The device includes a relay which may be coupled serially between the line and the apparatus or may function as a pilot relay for disconnecting the apparatus through, such as, a magnetic starter. The device has separate rectifying circuits which are coupled to the line or source voltage to provide a rectified source voltage reference. One of the rectified source voltage circuits is coupled to a gas-discharges tube through a variable resistor. This circuit also includes a potentiometer which provides a ripple factor or A.C. component to the rectified source voltage of this circuit. This circuit can be adjusted to select a plurality of voltage differentials proportionate to the source voltage at which the tube will fire and extinguish. The gas-discharge tube having specific breakdown and extinguishing voltage characteristics is coupled to control a pair of transistors connected in compound relation. The coil of the relay is coupled to one of the transistors. The other rectified source voltage drives the collector-emitter circuits and the coil. The coil is coupled, in parallel relation, with a diode and capacitor which provides a clamp across the coil. The capacitor also provides a time delay of relay acutation independent of the gas-discharge tube.
Other objects and features of the invention will be apparent from the following description and accompanying drawing which forms a part of this specification.